Recently, secondary batteries, such as a nonaqueous electrolyte secondary battery like a lithium ion secondary battery, have been actively researched and developed as a high energy-density battery. The secondary batteries, such as a nonaqueous electrolyte secondary battery, are anticipated as a power source for vehicles such as hybrid automobiles, electric cars, an uninterruptible power supply for base stations for portable telephones, or the like. Therefore, the secondary battery is demanded to, in addition to having a high energy density, be excellent in other performances such as rapid charge-discharge performances and long-term reliability, as well. For example, not only is the charging time remarkably shortened in a secondary battery capable of rapid charge and discharge, but the battery is also capable of improving motive performances in vehicles such as hybrid automobiles, and efficient recovery of regenerative energy of motive force.
In order to enable rapid charge-and-discharge, electrons and lithium ions must be able to migrate rapidly between the positive electrode and the negative electrode. However, when a battery using a carbon-based negative electrode is repeatedly subjected to rapid charge and discharge, precipitation of dendrite of metallic lithium on the electrode may sometimes occur, raising concern of heat generation or ignition due to internal short circuits.
In light of this, a battery using a metal composite oxide in a negative electrode in place of a carbonaceous material has been developed. In particular, in a battery using an oxide of titanium in the negative electrode, rapid charge and discharge can be stably performed. Such a battery also has a longer life than in the case of using a carbon-based negative electrode.
However, compared to carbonaceous materials, oxides of titanium have a higher potential relative to metallic lithium. That is, oxides of titanium are more noble. Furthermore, oxides of titanium have a lower capacity per weight. Therefore, a battery using an oxide of titanium for the negative electrode has a problem that the energy density is low. In particular, when a material having a high potential relative to metallic lithium is used as a negative electrode material, the voltage is lower than that of a conventional battery using a carbonaceous material. Therefore, when such a material is used for systems requiring a high voltage such as an electric vehicle and a large-scale electric power storage system, there is a problem that the number of batteries connected in series becomes large.
The potential of the electrode using an oxide of titanium is about 1.5 V (vs. Li/Li+) relative to metallic lithium. The potential of an oxide of titanium is attributed to the redox reaction between Ti3+ and Ti4+ upon electrochemical insertion and extraction of lithium, and is therefore electrochemically restricted. Conventionally, it has therefore been difficult to drop the potential of the electrode in order to improve the energy density.